Outlook Read only on Public calender

I have on some computers only read only rigths on other computers i can write, what ist the problem
Winodws 7 x64 outlook 2016 x32 with KOE

this is the mail comes from kopano

on 25.07.2017 at 08:37:50 you've tried to save a data in the folder 'Gemeinsamer Kalender - SYSTEM' on your device 'WindowsOutlook' ID: 'xxxxxxx'
This operation was not successful, as you lack write access to this folder.
Your data has been dropped and replaced with the original data on your device to ensure data integrity.
Below is a copy of the data you tried to save. If you want your changes to be stored permanently you should forward this email to a person with write access to this folder asking to perform these changes again.
Appointment
Timezone: xxxxx
Starttime: 26/07/2017 12:30:00
Subject: mvv
Endtime: 26/07/2017 13:00:00
Sensitivity: Normal
Busystatus: Busy
Alldayevent: No
Reminder: 15
Meetingstatus: Not a meeting
Body/Description: No changes made